1. Field of the Invention
This invention relates to a printed circuit boards and, more specifically, to edge connectors disposed at the periphery of printed circuit boards.
2. Description of the Related Art
Printed circuit boards of the type used in various computer hardware, e.g., modems, commonly include a copper conductor over which is applied a layer of nickel metal followed by a layer of gold. Because of the complexity of applying a layer of gold over a layer of nickel on a copper conductor, and the associated relatively high cost of gold, there exists a need for a replacement for the gold and nickel materials in edge connectors on printed circuit boards.
The blending of resins or adhesives with powdered conductive materials such as, gold, silver, copper, nickel or other metals or alloys, provides conductive pastes or inks. Such materials, also known as polymer thick films (ptf), generally fall into two classes. One class is the thermosetting kind (preferred for permanence) of which the epoxys, acrylics, and polyesters are examples. The other class is the thermoplastic kind (which soften when heated) composed of synthetic or natural polymer, (such as, polyimide siloxane, nylon, neoprene, or rubber) in a solvent or other suitable carrier.
For further information pertaining to conductive inks, conductive adhesive inks, adhesives and surface mount technology, in general, the reader is directed to Radio-Electronics, page 59 et seq., November, 1987, xe2x80x9cIntroduction to SMTxe2x80x9d, by Forrest M.
Conductive inks typically include metals such as silver, copper, lead or tin to provide electrical conductivity together with polymeric binders which provide adhesion.
Conductive inks have been applied by screen printing onto non-moldable insulating baseboard substances, such as FR4 or glass epoxy.
However, when printed circuit boards are manufactured by applying a conductive paste such as copper paste on the insulating base board by screen printing, there is generally inadequate adhesion of the conductive paste or ink to the circuit terminal portion. This is particularly pervasive when heat is applied to the board during the soldering of electrical components to the terminal portion.
Also, specific levels of resistivity are required for use as edge connectors in Peripheral Component Interconnect (PCI), Industry Standard Architecture (ISA), and Audio Modem Riser (AMR) buses.
The present invention provides edge connectors or edge contacts on printed circuit boards that overcome the disadvantages described above.
More specifically, the invention provides edge connectors or contacts in which the gold and nickel layers are replaced with a single layer of a conductive ink. The use of the conductive ink as an edge connector as described herein makes unnecessary the use of nickel plating between the upper layer of the connector and the copper conductor. Thus, not only is the use of gold avoided, but also the use of nickel, making the process more efficient and less expensive.
Thus, in one aspect, the invention provides edge connectors on a printed circuit board that are substantially permanent and do not crack or peal-off of the circuit board.
In another aspect, the invention provides printed circuit boards having at least one edge connector, where the edge connector comprises a copper conductor coated with a conductive ink where the ink generally has a sheet resistivity of about 0.1 to 0.5 xcexa9/square/15 xcexcm.
In a further aspect, the invention provides printed circuit boards having one or a plurality of edge connectors in which a copper conductor is coated with a conductive ink where the ink has a contact resistance of about 10xc3x9710xe2x88x923xcexa9 to 70xc3x9710xe2x88x923xcexa9.
The conductive ink suitable for use herein is a thermosetting resin containing graphite powder, carbon black and silver flakes. The silver loading is generally in the range of from about 10-50% by weight of the wet ink composition. The silver flakes suitable for use herein have a grind size not greater than about 10 xcexcm. Viewed another way, the silver loading is from about 50-75% by volume of the ink composition. Such an amount is sufficient to provide the edge contact with a sheet resistivity of from about 0.1 xcexa9/sq/15 xcexcm to 0.5 xcexa9/sq/15 xcexcm and a contact resistance of from about 10xc3x9710xe2x88x923 to 70xc3x9710xe2x88x923xcexa9.
While the inventive connector is particularly suitable for use in manufacturing ISA bus edge connectors, it may also be used in manufacturing PCI and AMR bus edge connectors.
The invention also provides methods for forming edge connectors on printed circuit boards comprising applying a conductive ink having a sheet resistivity of about 0.1 to about 0.5 xcexa9/sq/15 xcexcm over a copper conductor terminating at the edge of the circuit board. Generally, the conductive inks are applied to the circuit board using screen printing or other suitable technologies known to those skilled in the art.